Laser Metal Deposition (LMD) is an important Solid Freeform Fabrication (SFF) technology based on three-dimensional laser cladding. Similar to other processes such as Laser Engineered Net Shaping (LENS), Laser-Based Additive Manufacturing (LBAM) etc., LMD allows direct fabrication of functional metal parts directly from CAD solid models, as well as thin parts because the processing forces are low. It can also be used to repair parts; thus, reducing scrap and extending product service life.
In LMD, a laser beam is focused upon the surface of a substrate (workpiece) and generates a melt pool on the substrate. Metal powder is injected out through one or more delivery nozzles and into the focused laser beam. The conventional method of injection involves using high pressure inert gas such as argon to blow the metal powder out one or more delivery nozzles. The blown powder meets the laser beam and is absorbed and integrated into the melt pool, thus creating the “deposit” of the deposition process. The substrate is continually moved relative to the laser and powder injectors and layers are thereby added to the substrate. The blown-powder laser deposition process can produce fully-dense and metallurgically sound parts by this layered manufacturing method.
Many operational LMD quality control parameters depend on the characteristics of the gas-powder stream put out by the nozzles. Concentration of the powder is one such characteristic. The content of powder in the gas-powder stream has a large influence on the geometrical accuracy and the surface quality of the deposited buildup. FIG. 1 shows a schematic of a typical LMD system. The metallic particles injected into the laser beam are drawn from a feeding system through cylindrical inlets to the feeding nozzles. Insuring that the powder is efficiently and consistently fed to the powder delivery nozzles is critical to the LMD process.
Because the content of the gas-powder stream is an important factor in build quality, it is important that the powder be consistently and accurately metered into the carrier gas stream. Existing powder feeders, such as utilized in the system shown in FIG. 1, use mechanical structures to deliver metered powder flow. Typically, the metallic powder is drawn or pushed out of a powder reservoir via the action of a rotating wheel driven by a motor. Such mechanisms have certain deficits. In the first instance, mechanical powder delivery systems can create fluctuations in powder flow and often do not feed powder at a consistent rate. These fluctuations can create quality control issues in LMD systems that otherwise require consistent powder flow. Similarly, many existing mechanical powder feeding systems have difficulty producing a steady powder flow when low flow rates are required. This is primarily due to the fact that powder must be mechanically measured and dispensed.
Mechanical powder feeding systems also suffer from the fact that metal powder has a tendency to wear down and degrade anything with which it comes in motile contact. Additionally, mechanical powder feeding systems have another significant drawback that results from the fact that powder of any form will insert itself into any space, particularly those spaces between moving parts. In the case of metallic powders, such insertions often cause damage to moving parts. In view of the deficits of the prior art powder feeding devices, there is a need in the art for an improved powder delivery method and apparatus.